The invention relates to an efficient dredging system wherein continuous subaqueous excavation is maintained by delivering dredged material continuously to an assemblage of hopper barges connected together and to the dredging vessel in an assemblage through a system of pneumatic fenders acting on bow portions received in stern notches, the securing being effected by the pneumatic pressure applied by the fenders against the side hulls of the bow portions of the barges and dredging vessel. By removing the barges as an assemblage and quickly replacing same with a further assemblage of barges the dredging operation may continue without interruption. A prime mover having a bow portion similar to that of the dredging vessel may be utilized to push the detached barge assembly out to sea or otherwise where it may be dumped and thereafter to return the assemblage to the dredging vessel for replacing a like barge assemblage which has been filled with dredging material in the meanwhile.
The function of the contemporary hopper dredge is dredging, which is an underwater, subaqueous type excavation. The primary modes of operation of contemporary hopper dredging vessels are as follows:
1. Agitation Dredging -- The dredge spoils are allowed to overflow from the vessel's hoppers during the dredging operation. As a result, the vessel's hoppers need only be dumped at infrequent intervals. Unless the dredged material which overflows is carried away by the current, much and perhaps most of the overflow dredged material eventually is redeposited in the dredging project prism.
2. Side Casting -- In this mode of operation the dredging vessel continually discharges dredged material directly via an extended conduit to an immediately adjacent disposal area. As a practical matter, this mode requires that the disposal area be within about two hundred feet of the channel being dredged and parallel to such channel with a dike or the like isolating the disposal area from the dredging project prism.
3. Boom Dredging -- In this mode, the hopper dredge is located relatively near the disposal area but not sufficiently near to utilize the side casting mode. The hopper dredge proceeds to such disposal area whenever an economical load has been obtained in the dredge's hoppers and the dredged material is dumped in that portion of the disposal area nearest the point at which the hopper dredge left the dredging project. After such dumping, the hopper dredge resumes dredging at the point it previously left.
4. Dredging and Hauling -- In this mode, the disposal area is located at a substantial distance from the site being dredged. Each dredged load must therefore be hauled to the disposal area and dumped or otherwise discharged before the hopper dredge can return for another load. The disposal area may be relatively remote: that is three to sixty miles from the dredging site.
To accomplish the latter mode, Dredging and Hauling, and also the mode, Boom Dredging, the hopper dredge actually forms three separate functions which are: (1) The subaqueous excavation within the project prism: (2) The retention of solids and slurry or both which have been removed from the project prism, and (3) The transport and final disposition of the dredged and retained solids or slurry, together with return to the dredging site.
The contemporary hopper dredge is a unique self-propelled, seagoing ship which is physically designed, constructed and operated to perform all three of the above-mentioned functions. It will be appreciated that the hopper capacity limitations of the dredge requires that these functions be performed in the sequence given. It has been found that for a typical 3,000 cubic yard hopper capacity trailing section hopper dredge wherein the dredging project is located approximately 6 miles from the sea-dump disposal site, the ratio between dredging and retention activity and transport and disposal activity is approximately one to two. Accordingly, if a hopper dredge of such type dredges say 8,500,000 cubic yards in 300 operating days and the vessel's operating cost, excluding repairs, during such period is about $3,000,000, about $1,000,000 is applied to dredging and retention activities and about $2,000,000 to disposal activity.
Under the same conditions, except that the sea dump disposal site is located 30 miles from the dredging site, the ratio between the dredging and retention activity on one hand and the transport and disposal activity on the other, is about 1 to 9.
At present, due to increased interest in geological and ecological considerations, there is a trend leading to a substantial increase in distance between the dredging activity and the disposal dump and an increased use of the dredging and hauling mode of operations. The alternative is to provide specialized disposal facilities for the direct or indirect discharge of dredging materials at shoreside establishments on each project. In either event, the expenditures for the dredging and disposal operations are greatly increased.
In face of the increasing costs, an expansion of dredging operations is and will be for some time required for commerce and traffic in the various harbors and inland waterways of this country. This is particularly so due to the substantial increases in draft and capacity of merchant vessels which has taken place in the last several years.
Although a growing awareness concerning marine and shoreside ecological considerations has served to emphasize the economical problems of present hopper dredger operations, the fact is that a need has long existed for a system which is substantially more efficient than that used at present from the standpoint of productivity in the excavation and disposal of the dredged material.